It is known that packing containers of the non-returnable type for milk, juice and other liquid foods are manufactured from flexible packing laminate which usually comprises layers of fibrous material, thermoplastics and possibly metal foil. Packing machines which manufacture this type of packing container often use a principle of manufacture which involves the conversion in the first place of the weblike packing material to a liquid-tight tube with a longitudinal sealing joint. This tube extends substantially downwards through the machine and is filled continuously with liquid contents up to a certain predetermined level. Below this predetermined level, a conversion of the filled tube to individual, closed packing containers is carried out with the help of external sealing and forming devices, which packing containers, in the process, become wholly filled with contents. The contents are fed to the tube via a filling pipe extending concentrically through the tube which via valves is connected to a contents reservoir situated above. At the lower end of the filling pipe there is a valve which is joined via a mechanical linkage system to a float arranged around the filling pipe. When the contents flow out into the packing material tube and reach the float, the same is lifted, which means that the valve is closed so that the feed of contents is cut off. This arrangement provides a satisfactory level control in the majority of types of contents up to now, which as a rule are constituted of relatively mobile liquids such as milk, juice, wine or the like. If it is desired to use contents of a higher viscosity, such as cream, ice-cream or butter, however, this arrangement cannot be used, since such contents cannot be fed to the tube through gravity but have to be pumped in and would, moreover, penetrate past the float and prevent the movement of the same as well as cause a cleaning problem. It would be desirable, therefore, to provide an arrangement for the control of the flow of contents in the abovementioned type of packing machines when contents of relatively high viscosity are to be packaged.